This invention relates to the field of nuclear magnetic resonance (NMR) spectroscopic devices. NMR spectroscopy is an analytical and diagnostic technique that can be used for the structural and quantitative analysis of a compound in a mixture. NMR is based on the nuclear magnetic properties of certain elements and isotopes of those elements. It is based on the principle that nuclei with a non-zero spin will have a magnetic dipole and therefore will interact with electromagnetic (EM) radiation.
The presence or absence of a spin and the nature of this spin is expressed in terms of the spin quantum number of the nucleus, which may either be 0, ½ or multiples of ½.
In a uniform magnetic field a nucleus having a spin quantum number of ½ may assume two orientations relative to the applied magnetic field. The two orientations have different energies so that it is possible to induce a nuclear transition by the application of electromagnetic radiation of the appropriate frequency. This transition is resonance. Resonance arises when the correct combination of magnetic field strength and exciting frequency characteristics of the nuclei of interest are applied.
After resonance is achieved the NMR instrument records a signal, the signal being a function of the nature and amount of a compound of the test sample as well as nuclear magnetic relaxation considerations.
An NMR spectrometer generally comprises one or more magnets producing a strong homogenous field within a test region. The size and complexity of NMR spectrometers are largely a function of the magnetic field requirements.
Conventional NMR requires a laboratory electromagnet, or superconducting magnet. The spectroscopic information is obtained by using uniform magnetic fields and thus the technique is inherently invasive as the field uniformity is restricted to small volumes and materials must be placed inside the magnet system.
An alternative design is the “inside out” NMR which uses open magnet designs for measurements in the field without sample size restrictions. However a disadvantage of such a design is reduced sensitivity and lack of resolution and the field is less uniform than in such a magnetic configuration.
A development beyond the use of inside out NMR relates to the development of mobile NMR devices which have been developed for analysis of many things including oil wells, water reservoirs, plant growth and life cycles and moisture detection (for example in wood or concrete).
There are a number of difficulties associated with the development of small scale inside NMR apparatus. These include:                1) the homogeneity of the magnetic field;        2) it is important to provide a magnet with the highest field possible;        3) such a strong magnetic field generally requires larger, heavier magnets, which increases the costs of the spectrometer;        4) with such a magnet configuration typically only the surface area of a subject can be analysed by the technique.        
A number of parties have applied themselves to these difficulties in the development of small scale NMR apparatus.
U.S. Pat. No. 6,163,154, (Anderson et al), discusses the development of small scale NMR apparatus for the measurement of a patient's glucose levels. It employs a pair of opposed permanent magnets and a plurality of annual circular magnets which are cancelling magnets.
U.S. Pat. No. 6,081,116, (Wu et al), deals with NMR apparatus for geological applications and employs a plurality of cylindrical magnets to approximate a permanent ring magnet. This will reduce the cost of the use of a single ring magnet.
U.S. Pat. No. 5,959,454, (Westphal et al), deals with the magnet arrangement for an NMR tomography system for skin and surface examinations. This is a one sided NMR system having two ring magnets and a cylindrical magnet the locations being so as to impart a certain degree of uniformity.
A number of people have alternatively attempted to deal with the problem that in one sided NMR, often only the surface region of the sample is analysable due to magnetic field concerns.
U.S. Pat. No. 5,739,688, (Krieg), attempts to profile in the z-axis direction (into the sample), by employing a static magnetic field having a predetermined inhomogeneity in the z direction. It uses slices perpendicular to the direction of inhomogeneity (z axis) with operating the apparatus by a pulse sequence with shortened measurement time. This allows for excitation of one slice independent of another, and overcomes relaxation disadvantages.
U.S. Pat. No. 5,126,674, (Miller et al), again deals with a one sided NMR apparatus and the technique of planar imaging. It creates an inhomogeneous magnetic field and the RF frequency selection excites only one “volume of interest” at a time. Again there is no need for relaxation as each is excited independently of the other.
U.S. Pat. No. 4,528,509, (Radda et al), and U.S. Pat. No. 4,710,713, (Strikman), both similarly deal with three dimension imaging via a homogenous field in the z-axis direction.